auto merge of #7856 : brson/rust/no-thread-per-core, r=pcwalton

This doesn't make sense under the new scheduler.

diff --cc src/libstd/task/mod.rs
Simple merge

diff --cc src/libstd/task/rt.rs
Simple merge

diff --cc src/libstd/task/spawn.rs
Simple merge

diff --cc src/rt/rust_builtin.cpp
Simple merge

diff --cc src/rt/rustrt.def.in
Simple merge